Currently, most cities in the United States are faced with a severe shortage of natural gas that is curtailing industrial operation and expansion, reducing the fuel supplies of institutions such as schools and hospitals and forcing new home construction to utilize expensive electrical energy for heating and cooking which results not only in higher prices but also causes a very low resource utilization efficiency. Because of a continuing decline in the production of natural gas, this crisis will become more severe and even become felt in Canadian metropolitan areas as their growth and energy consumption exceeds the available natural gas supply. In anticipation of this problem, Canada is already reducing exports of natural gas to the U.S. and seriously considering gasification of coal in the western provinces to reduce the impact of the natural gas shortage. Ironically, in the metropolitan areas, where the gas shortages are most severe, the disposal of an ever increasing volume of solid wastes from municipal, industrial and sewage-treatment operations is also a growing problem of major proportions.
Conversion of this solid waste into a synthetic natural gas in an environmentally acceptable fashion would, therefore, assist in the solution of two very severe urban problems. Conversion of solid waste into a fuel gas of much lower Btu than natural gas is possible with known processes and apparatus but production of synthetic natural gas which can be substituted for natural gas has not been commercially practical heretofore. The only process currently available for converting solid wastes into synthetic natural gas is by biological digestion. The two main problems with biological digestion are:
1. The long solid waste residence time requiring extremely large vessel capacity if large volumes of waste are to be treated.
2. Disposal of the by-product sludge from the process.
The invention described here offers the following advantages over prior processes:
1. No preseparation of metal and glass is required.
2. Higher yields of methane are produced than are possible by alternate technology.
3. Gas is produced at the proper pressure for purification.
4. The recycle value of the metal and glass is preserved because no oxidation or sintering occurs.
5. Relatively small volume of disposable residue.
General background to the technology is given in U.S. Pat. Nos. 3,733,187 and 3,729,298. For the purpose of this invention, the two patents are incorporated by reference as background information and showing the state of the art.
It is the intention of this disclosure to teach how to take solid wastes from municipal garbage and trash disposal systems, sewage treatment systems, industrial waste collection, etc. and convert the same into usable hydrocarbons on a practical basis with a minimum of ash to be disposed of at the end of the process. This invention also teaches how to greatly increase the yield of methane per unit of waste over that achieved using prior-art conversion systems.
While there are a wide variety of possible individual chemical constituents in the infinite variety of solid waste materials enumerated, the fact is that as a total group comingled in the ordinary course of transportation to a fill site or treatment plant, the distinction in quantities of chemical composition is somewhat limited. For the most part, the solid waste includes large portions of carbon and considerable oxygen and hydrogen. It is a prime objective of this invention to convert the carbon to usable gaseous hydrocarbons.
It is well known that contacting solid waste with hydrogen containing gas will convert certain of the carbon compounds to methane, ethane, and perhaps other hydrocarbons. This invention illustrates:
1. That it is advantageous to physically separate the methane production reactor from the gasifier reactor in which the hydrogen containing synthesis gas is produced. The methane production reactor being a confined zone under pressure where solid waste is contacted with hydrogen containing gas. The gasifier reactor being a chamber under pressure where carbonaceous residue (called char) from the confined zone is contacted with steam and gaseous oxygen.
2. That it is desirable to control the degree of carbon conversion in the methane production reactor below a critical level to insure that sufficient carbonaceous char is delivered to the gasifier reactor. With proper quantities of char delivered to the gasifier reactor the synthesis gas produced therein will have enough sensible heat to bring the solid wastes to reaction temperature (when the synthesis gas is delivered to the methane production reactor).
3. There is a point of steady state operation within the system at given solids, steam and oxygen feed rates, system pressure, and types of methane production reactors and gasification reactors where almost all carbon in the waste is consumed or converted during the process.
4. The point of steady state operation shifts upon modification of one or more of the variables enumerated above to another steady state point.
5. The yield of methane and consumption of oxygen per unit of solid waste treated depend on this steady-state operation point. In general, the closer operation in the methane production reactor is to maximum carbon conversion levels, the higher will be the methane yield and the lower the oxygen consumption.
It is believed that carbon compounds in solid waste become subject to cracking in the confined zone under pressure and temperature, and that with hydrogen gas available, the cracking carbon molecules tend to react with the hydrogen to form methane. Whereas, in the absence of hydrogen, the tendency is to polymerize and form longer carbon chains with an increase in the formation of hydrocarbon gum and tar. It has been experimentally determined that a pressure of about 18 atmospheres is satisfactory to achieve the degree of conversion desired and to produce a gas that is both rich in methane and low in tars. At this pressure, reactor vessels may be economically constructed and the cost of the subsequent purification of the product gas from the methane production reactor is much reduced over what it would be at atmospheric pressure. In addition, the feeding of the solid wastes into the confined zone by the use of conventional lock hoppers can be accomplished at this pressure. Therefore, the major portion of the experimental work upon which this concept is based was carried out close to 18 atmospheres pressure.
Given this parameter, the temperature of the reaction chamber becomes important because at too high a temperature the methane produced tends to be cracked into carbon and hydrogen. It was discovered that a satisfactorily high yield of methane can be achieved in the methane production reactor at bulk temperatures of 900.degree. C although the hydrogen containing gas mixture can be fed into the confined reaction zone at much higher temperatures because thermal equilibrium will be reached very rapidly between the cooler waste and the hot hydrogen containing gas. The carbon of the solid waste is partially converted to methane and small quantities of other low molecular weight hydrocarbons in the confined zone and the gaseous mixture is drawn off to a cleaning and purifying zone where carbon dioxide is removed and additional methane is formed by the reaction of the excess carbon monoxide and hydrogen in the product gas. After this treatment the resulting gas can be used as a replacement for natural gas.
Carbon containing char is discharged from the confined zone after the partial conversion to fall through a generally vertical duct toward a quenching bath of water. A side branch off the vertical duct is arranged to receive a transversely directed jet of steam which will carry along with it the lighter weight solid waste (mostly char) while the heavier solid waste (usually glass and metal) will fall on downward toward the quenching water bath where the heat contained in the metal and glass will be recovered by conversion to steam which drifts upward countercurrent to the falling glass and metal.
The char blown into the side branch by the jet of steam is directed to a reaction chamber and oxygen is injected simultaneously into the chamber. The combination of carbon containing char, steam and oxygen at a pressure of approximately 18 atmospheres causes well known gasification chemical reactions to occur with the resulting formation of synthesis gas consisting essentially of carbon monoxide, carbon dioxide, hydrogen gas and some leftover water vapor. Overall, the gasification reactions are exothermic but require a temperature above 900.degree. C for reaction to occur at a commercially reasonable rate. The heat is supplied by combustion of part of the carbon with oxygen. Hot synthesis gas is withdrawn from the gasifier and delivered to the methane production reactor to provide the hydrogen containing gas used for the methane forming reactions. The ash residue remaining in the reaction chamber following the gasification is only a very small percentage of the net weight of the solid waste initially delivered to the confined zone.